© 2010 pearson education, inc. chapter 24 life in the universe

© 2010 Pearson Education, Inc.

Chapter 24Life in the Universe


24.1 Life on Earth

Our goals for learning:• When did life arise on Earth? • How did life arise on Earth?• What are the necessities of life?


When did life arise on Earth?


Earliest Life Forms

• Life probably arose on Earth more than 3.85 billion years ago, shortly after the end of heavy bombardment.

• Evidence comes from fossils, carbon isotopes.


Fossils in Sedimentary Rock

• Relative ages: deeper layers formed earlier

• Absolute ages: radiometric dating


Fossils in Sedimentary Rock

• Rock layers of the Grand Canyon record 2 billion years of Earth’s history.


Earliest Fossils

• The oldest fossils show that bacteria-like organisms were present over 3.5 billion years ago.

• Carbon isotope evidence dates the origin of life to more than 3.85 billion years ago.


The Geological Time Scale


How did life arise on Earth?


Origin of Life on Earth

• Life evolves through time.

• All life on Earth shares a common ancestry.

• We may never know exactly how the first organism arose, but laboratory experiments suggest plausible scenarios.


The Theory of Evolution

• The fossil record shows that evolution has occurred through time.

• Darwin’s theory tells us HOW evolution occurs: through natural selection.

• Theory supported by discovery of DNA: evolution proceeds through mutations.


Tree of Life

• Mapping genetic relationships has led biologists to discover this new “tree of life.”

• Plants and animals are a small part of the tree.

• Suggests likely characteristics of common ancestor


• These genetic studies suggest that the earliest life on Earth may have resembled the bacteria today found near deep ocean volcanic vents (black smokers) and geothermal hot springs.


Laboratory Experiments

• The Miller-Urey experiment (and more recent experiments) show that the building blocks of life form easily and spontaneously under the conditions of early Earth.


Microscopic, enclosed membranes or “pre-cells” have been created in the lab.


Chemicals to Life?


Could life have migrated to Earth?

• Venus, Earth, Mars have exchanged tons of rock (blasted into orbit by impacts).

• Some microbes can survive years in space.


Brief History of Life

• 4.4 billion years - early oceans form• 3.5 billion years - cyanobacteria start releasing

oxygen• 2.0 billion years - oxygen begins building up in

atmosphere• 540–500 million years - Cambrian Explosion• 225–65 million years - dinosaurs and small

mammals (dinosaurs ruled)• Few million years - earliest hominids


Thought QuestionYou have a time machine with a dial that you can spin to send you randomly to any time in Earth’s history. If you spin the dial, travel through time, and walk out, what is most likely to happen to you?

A. You’ll be eaten by dinosaurs.B. You’ll suffocate because you’ll be unable to

breathe the air.C. You’ll be consumed by toxic bacteria.D. Nothing. You’ll probably be just fine.


Origin of Oxygen

• Cyanobacteria paved the way for more complicated life forms by releasing oxygen into atmosphere via photosynthesis.


What are the necessities of life?


Hardest to find on other planets

Necessities for Life

• A nutrient source

• Energy (sunlight, chemical reactions, internal heat)

• Liquid water (or possibly some other liquid)


What have we learned?

• When did life arise on Earth?– Life arose at least 3.85 billion years ago,

shortly after end of heavy bombardment.

• How did life arise on Earth?– Life evolved from a common organism

through natural selection, but we do not yet know the origin of the first organism.

• What are the necessities of life?– Nutrients, energy, and liquid water.


24.2 Life in the Solar System

Our goals for learning• Could there be life on Mars? • Could there be life on Europa or other jovian

moons?


Could there be life on Mars?


Searches for Life on Mars

• Mars had liquid water in the distant past.• Still has subsurface ice; possibly subsurface water near sources of volcanic heat


In 2004, NASA Spirit and Opportunity rovers sent home new mineral evidence of past liquid water on Mars.


The Martian Meteorite debate

Composition indicates origin on Mars.

• 1984: meteorite ALH84001 found in Antarctica • 13,000 years ago: fell to Earth in Antarctica• 16 million years ago: blasted from surface of Mars• 4.5 billion years ago: rock formed on Mars


• Does the meteorite contain fossil evidence of life on Mars?

Most scientists are not yet convinced.


Could there be life on Europa or other jovian moons?


• Ganymede, Callisto also show some evidence for subsurface oceans.

• Relatively little energy available for life, but there still may be enough.

• Intriguing prospect of THREE potential homes for life around Jupiter alone.

Ganymede Callisto


Titan

• The surface is too cold for liquid water (but there may be some deep underground).• Has lakes of liquid ethane/methane on its surface.


Enceladus

• Ice fountains suggest that Enceladus may have a subsurface ocean.



• Could there be life on Mars?– Evidence for liquid water in past suggests that

life was once possible on Mars.

• Could there be life on Europa or other jovian moons?– Jovian moons are cold but some show

evidence for subsurface water and other liquids.


24.3 Life around Other Stars

Our goals for learning:• Are habitable planets likely?• Are Earth-like planets rare or common?


Are habitable planets likely?


Habitable Planets

Definition:

• A habitable world contains the basic necessities for life as we know it, including liquid water.

• It does not necessarily have life.


Constraints on star systems:

1) Old enough to allow time for evolution (rules out high-mass stars - 1%)

2) Need to have stable orbits (might rule out binary/multiple star systems - 50%)

3) Size of “habitable zone”: region in which a planet of the right size could have liquid water on its surface

Even with these constraints, billions of stars in the Milky Way could potentially have habitable worlds.


The more massive the star, the larger its habitable zone— and the higher probability of a planet existing in this zone.


Finding them will be hard

Recall our scale model solar system:

• Looking for an Earth-like planet around a nearby star is like standing on the East Coast of the United States and looking for a pinhead on the West Coast—with a VERY bright grapefruit nearby.

• But new technologies will allow us to search for such planets.


• Kepler (launched in 2009) will monitor 100,000 stars for transit events for 4 years.

Later: SIM and TPF interferometers may obtain spectra and crude images of Earth-size planets.


Spectral Signatures of Life

Earth

Venus

Mars

Oxygen/ozone


Are Earth-like planets rare or common?


Elements and Habitability

• Some scientists argue that the proportions of heavy elements need to be just right for the formation of habitable planets.

• If so, then Earth-like planets are restricted to a galactic habitable zone.


Impacts and Habitability

• Some scientists argue that Jupiter-like planets are necessary to reduce rate of impacts.

• If so, then Earth-like planets are restricted to star systems with Jupiter-like planets.


Climate and Habitability

• Some scientists argue that plate tectonics and/or a large moon are necessary to keep the climate of an Earth-like planet stable enough for life.


The Bottom Line

We don’t yet know how important or negligible these concerns are.



• Are habitable planets likely?– Billions of stars have sizable habitable zones,

but we don’t yet know how many have terrestrial planets in those zones.

• Are Earth-like planets rare or common?– We don’t yet know because we are still trying

to understand all the factors that make Earth suitable for life.


24.4 The Search for Extraterrestrial Intelligence

Our goals for learning:• How many civilizations are out there?• How does SETI work?


How many civilizations are out there?


The Drake EquationNumber of civilizations with whom we could potentially

communicate

= NHP flife fciv fnow

NHP = total number of habitable planets in galaxy

flife = fraction of habitable planets with life

fciv = fraction of life-bearing planets with civilization at some time

fnow = fraction of civilizations around now


We do not know the values for the Drake equation.

NHP : probably billions

flife : ??? hard to say (near 0 or near 1)

fciv : ??? took 4 billion years on Earth

fnow : ??? depends on whether civilizations can survive long-term


Are we “off the chart” smart?

• Humans have comparatively large brains.

• Does that mean our level of intelligence is improbably high?


How does SETI work?


SETI experiments look for deliberate signals from extraterrestrials


We’ve even sent a few signals ourselves…

Earth to globular cluster M13: Hoping we’ll hearback in about 42,000 years!


Your computer can help! SETI @ Home: a screensaver with a purpose



• How many civilizations are out there?– We don’t know, but the Drake equation gives

us a framework for thinking about the question.

• How does SETI work?– Some telescopes are looking for deliberate

communications from other worlds.


24.5 Interstellar Travel and Its Implications to Civilization

Our goals for learning:• How difficult is interstellar travel?• Where are the aliens?


How difficult is interstellar travel?


Current Spacecraft

• Current spacecraft travel at <1/10,000c; 100,000 years to the nearest stars

Pioneer plaque Voyager record


Difficulties of Interstellar Travel

• Far more efficient engines are needed.• Energy requirements are enormous.• Ordinary interstellar particles become like cosmic rays.• Social complications of time dilation.


Where are the aliens?


Fermi’s Paradox

• Plausible arguments suggest that civilizations should be common. For example, even if only 1 in 1 million stars gets a civilization at some time 100,000 civilizations

• So why we haven’t we detected them?


Possible solutions to the paradox1) We are alone: life/civilization is much rarer

than we might have guessed.• Our own planet/civilization looks all the more

precious…


2) Civilizations are common, but interstellar travel is not because: interstellar travel is more difficult than we think. the desire to explore is rare. civilizations destroy themselves before achieving

interstellar travel.

These are all possibilities, but not very appealing…

Possible solutions to the paradox


3) There IS a galactic civilization…

… and some day we’ll meet them.

Possible solutions to the paradox



• How difficult is interstellar travel?– Interstellar travel remains well beyond our current

capabilities and poses enormous difficulties.

• Where are the aliens?– Plausible arguments suggest that if interstellar

civilizations are common then at least one of them should have colonized the rest of the galaxy.

– Are we alone? Has there been no colonization? Are the colonists hiding? We don’t know yet.

© 2010 pearson education, inc. chapter 24 life in the universe

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